"It's very atmospheric and mysterious, but the mystery doesn't give the players any clues to solve it. In the end, if they do the right thing it will by chance ..."

And:

"the "winning" scenario is damn near impossible."

I am sad to say these reviews were wrong. I was quite unable to execute a TPK. Not only that, all the PCs survived mostly unharmed and with their macguffin in hand!

Spoilers follow.

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The Set Up
Because I'm lazy sometimes, I followed the instructions on the set up:

If this adventure is a part of a campaign (LR: well, if you can call my improvisations a campaign, sure): feed the players stories of inconceivable wealth hidden in the mountain, inside the shrine of an ancient death cult. If the players are in search of a special book, sword or other storied trinket it is rumored to be there (and is—probably in Area 22—but don't tell them that yet).

Ok, so ... the PCs have killed the pepper pot defense units (daleks), taken over Facility Zero, activated the Cryogi®™, and found out that to fully activate it they need to refuel the Facility Main Core. After successfully merging personalities with the mummified remains of Jane Smith, the Facility Administrator, in the Modified Ur-Reality Development Expansion Recreator™, Adobe Suttle accessed the lower level Builder Subroutine, which identified a location where an intact radiothermal barrel had been stored.

In the Mouths of the Meat Mountain of Madness™.

I believe the name of the location successfully alerted the PCs to the ridiculous potential lethality of this location.

I also outlined the whole adventure for myself to help run it in my notebook:

The Approach & The Cabin

The Shrine. Notice the Thumbworm of Doom.

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The Play Through
DFD has a rather simple structure: Approach / Cabin / Shrine.

The Approach, which can be run as pretty much atmospheric. I added some random encounter tables, including a snow demon in the snow because Longwinter 2. The PCs ran from all encounters (saving those precious, rare hps), treated the rustic PC very kindly and generally did fine here.

The Cabin is a creepy, haunty cabin. The PCs mucked around with everything, and got useful information from much that was there. They were generally careful and nobody died terribly. The purple lotus powder was very popular. But again, nobody died. Hint: the painting shows the altar, which is where the holy radiothermal barrel was also displayed! With a pictomancer who can pass through paintings / walk into paintings, this was really harvested for information. The mad wizards also took samples of all the liquids and strange things they found for "later study".

The Shrine is the biggest part of the adventure and home of the death cult. It thematically splits into three interconnected sections: the cathedral and priest quarters (but the priests are long dead), the tombs of the greater dead (with lots of role-play opportunities here), and the crypts of the thousands of cult sacrifices.

First comes the cathedral and quarters. All the skulls and creepiness do give the game away, but the organ is a great toy and temptation a great mistress. Quinn and Todd played with it a lot, and ended up ageing 20 years each. Salami stole the offerings and was cursed with a disadvantage on all attack rolls. They explored the quarters thoroughly and found the connecting shaft to the tombs.

In the tombs they discovered the interrogator, and dispatched it, then next opened the tomb of the architect. There they had long discussions with the architect and learned about the praetor-pontifex. This was enough for them and they decided to first clear out the cathedral, while temporarily blocking the shaft.

After carefully going through everything that was left, they entered the crypts. There they found the the countless mummies. They methodically burned the priest and warrior mummies, collecting the melted jewellery afterwards. Going forward, they came to the parasite and became very, very careful. Quinn matched the parasite's song and Doc Odd gingerly removed the radiothermal barrel from the altar using the floating cheshire cat. They also tested one of the globes, and found it awakened a commoner mummy.

At this point, they sealed the commoner crypts and walked away.

And that was that! One interrogator killed and a couple of thousand sleeping mummies burned. One radiothermal barrel recovered.

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The Summary
DFD is one large, scary, set bear trap. Everything else in the adventure is predicated on the PCs poking stuff.

A careful group that doesn't treat each adventure as a slaughter-yard / monster-hunting expedition should do fine. There are no "trick" gotchas - right from the get-go, the adventure makes clear how lethally dangerous things _can_ be. If a PC ignores the skulls, the curses, the warnings, and still picks up protected items, well, a PC suffers the effects.

OK, there is one "trick" in the dungeon that isn't explained well for players: the big skull countdown timer. It's not actually linked to something coming alive and hurting them, but to how much time they will have to escape once / if they awaken the dead horde.

When running the adventure, I was clear in the descriptive warnings and up front about risks, e.g., "the skull altar looks menacing," "the black water is dank and has an evil smell," "curses and warnings are depicted on the walls", "walking over the sleeping parasites looks like it will be very hard and there are so many spheres you might crush some of them," "the ice around the mummies of the warriors, in this second crypt, has already melted a bit more than the ice around the priest mummies."

Overall, I had a lot of fun running DFD. I think the players did, too.

One thing to keep in mind: if you have a game system that makes it hard to create new characters, neither the referee nor the players will want to see characters die! This is bad! Make character generation faster, and the games will be more fun!

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Poking At Details

the map should have page numbers on it for faster reference.

white text on dark background doesn't leave space for notes.

I lost the printed out handout maps at some point.

I'll think of some more, but those are it for the moment.

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The System
I run a 5E D&D game - more or less. You can find more about it here. I don't use backgrounds, bonds, inspiration, feats (mostly), and limit the maximum power range of characters to about levels 5–6. Individual spells, items, abilities, can exceed this by a lot, but hit points are generally low. I generally use morale (2d6), re-roll initiative each round, more lethal death rules (on 0 save or die, option for healer to rush in and use heal check instead of failed save), longer rests.

I had no problems converting the module on the fly.

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Dramatis Personae (characters in italics present for most sessions).Doc "Odd" Todd the Dentist Wizard / Mad Scientist, his Stage Coach, his Cheshire Cat Demon Familiar,
Salami Rocquefort the Gunslinger / Rogue, his henchman "Hi" John the Weed Cultist,Adobe Suttle the Pictomancer AND Jane Smith, Facility Administrator Purple ClassQuinn Medicine Warrior the Warrior, her Goat Crystal Stardust, her Cat-Xenomorph Splice Jones
The Finisher, Saloon Brawler and Martial Artist,
Lem Goh, the Dwarf Technomancer who believes himself a golem,
Rod the Speed Freak / Fighter,
a few others that I forget right now ...